The present invention pertains to powder metallurgy and the preparation of ferrous metal powders and the use of such powders to produce near net shape components.
Powder metallurgy is becoming increasingly important these days for producing a variety of simple- and complex-geometry near net shape carbon steel components for the automobile and appliances industries. These components require close dimensional tolerances, good strength and surface properties such as hardness and wear resistance. Manufacture of the components involves pressing metal powders that have been premixed with graphite and organic lubricants into useful shapes, generally is referred to as as-pressed or green components, and then sintering the shaped components at high temperatures in a batch or continuous furnace in the presence of a controlled atmosphere. The sintered components can then be used as is or given minor surface finishing.
Carbon steel powder metal components are generally produced from metal powders containing a mixture of iron and graphite, which is added to the powder to provide strength, increase surface hardness, and control the dimensions of sintered components. The components pressed from these powders can be sintered in a continuous furnace operated above about 2,000.degree. F. (1093.degree. C.) in the presence of a controlled atmosphere containing primarily a mixture of nitrogen and hydrogen. The amount of hydrogen present in the atmospheres varies between 2 to 15% depending upon the source and supply mode of the atmospheres.
The controlled atmospheres used for sintering carbon steel components are generally endothermic (produced by endothermic generators), pure nitrogen blended with endothermic generated atmosphere, dissociated ammonia or pure hydrogen. The endothermic atmospheres are produced by catalytically combusting a controlled amount of hydrocarbon gas, such as natural gas in air in endothermic generators. The endothermic atmospheres typically contain nitrogen (.about.40%), hydrogen (.about.40%), carbon monoxide (.about.20%), and impurities in the form of carbon dioxide, moisture and unreacted hydrocarbon gas. The atmospheres produced by dissociating ammonia contain hydrogen (.about.75%), nitrogen (.about.25%) and impurities in the form of moisture and unconverted ammonia.
The use of endothermically generated atmospheres for sintering carbon steel components have been known to cause undesirable cyclic carburization and decarburization of sintered components due to the presence of high levels of carbon monoxide, hydrogen and moisture. Therefore, endothermically generated atmospheres by themselves are rarely used to produce carbon steel components requiring close dimensional tolerances, good strength and consistent surface properties such as hardness and wear resistance. These atmospheres are, therefore, mixed with pure nitrogen to reduce (1) effective concentrations of carbon monoxide, hydrogen and moisture and (2) undesirable cyclic carburization and decarburization. For example, 20% of endothermically generated atmosphere is mixed with 80% nitrogen to provide an effective hydrogen concentration of about 8%, the resulting atmosphere used to produce carbon steel components with consistent quality and properties.
The use of dissociated ammonia atmospheres by themselves for sintering carbon steel components have been known to severely decarburize sintered components due to the presence of high levels of hydrogen. Therefore, these atmospheres alone are not used for sintering carbon steel components. They are mixed with pure nitrogen to reduce the effective concentration of hydrogen to about 12% prior to being used for sintering carbon steel components.
The presence of 8 to 12% hydrogen in blends of nitrogen and endothermic atmospheres and nitrogen and dissociated ammonia atmospheres often decarburizes surfaces of sintered components, thereby reducing their surface hardness and wear resistance. The extent of reduction in surface hardness or wear resistance varies with the amount of hydrogen present in these atmospheres. Because of these variations, it is difficult to pick the right source and supply mode of these atmospheres for sintering carbon steel components and produce sintered components with consistent quality and properties.
A small amount of an enriching gas such as natural gas, or any other hydrocarbon gas, can be added to these atmospheres to counter the decarburization effect of hydrogen. However, the selection of an improper amount of an enriching gas results in forming soot in the furnace and carburizing furnace components such as the muffle and belt, thus reducing their useful life. Therefore, there is a need to (1) reduce variations in the physical properties of sintered carbon steel components due to variation in the amount of hydrogen in the atmosphere and (2) produce carbon steel components with consistent quality and properties.
A number of workers in the field have proposed adding copper to iron-graphite powders to increase the properties of the pressed and sintered parts. For example R. L. Lawcock and T. J. Davies in a technical paper titled "Effect of Carbon on Dimensional and Microstructural Characteristics of Fe--Cu Compacts During Sintering" describe sintering of Fe--Cu--C compacts containing 1% or more of copper in 75% hydrogen and 25% nitrogen atmosphere. A technical paper by N. Dautzenberg and H. J. Dorweiler titled "Dimensional Behavior of Copper-Carbon Sintered Steels" describes sintering of Fe--Cu--C compacts containing 1% or more copper. J. M. Torralba, L. E. G. Cambronero and J. M. Ruiz in their paper titled "Influence of the Nature of Powders on Properties and Microstructure of Sintered Cu and Ni Steels" describe sintering of Fe--Cu--C compacts containing 2% or more of copper. C. Durdaller, describes sintering of Fe--Cu--C compacts containing 2% or more of copper in his publication titled "The Effect of Additions of Copper, Nickel and Graphite on the Sintered Properties of Iron-Base Sintered P/M Parts." Y. Trudel and R. Angers published a paper titled "Comparative Study of Fe--Cu--C Alloys Made From Mixed and Prealloyed Powders" in which they describe sintering of Fe--Cu--C compacts containing 2% or more of copper. Another paper by Y. Trudel and R. Angers titled "Properties of Iron Copper Alloys Made from Elemental or Prealloyed Powders" describe sintering of Fe--Cu--C compacts containing 1% or more of copper. In a technical paper by S. J. Jamil and G. A. Chadwick titled "Investigation and Analysis of Liquid Phase Sintering of Fe--Cu and Fe--Cu--C Compacts" the authors describe sintering of Fe--Cu and Fe--Cu--C compacts containing 10% copper.
None of the prior art workers disclosed Fe--Cu--C powders or parts made therefrom where the copper context was less than 1% by weight and the effects of atmosphere compositions on the as sintered part, especially on surface properties of the finished parts.